Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The amount of radiation and its placement must be accurately controlled to ensure both that the tumor receives sufficient radiation to be destroyed, and that the damage to the surrounding and adjacent non-tumorous tissue is minimized.
In external source radiation therapy, a radiation source external to the patient treats internal tumors. The external source is normally collimated to direct a beam only to the tumorous site. The source of high energy radiation may be x-rays, or electrons from linear accelerators in the range of 2–25 MeV, or gamma rays from highly focused radioisotopes such as a Co.sup.60 source having an energy of 1.25 MeV.
One form of external radiation therapy uses the precision of a computed tomography (CT) scanner to irradiate cancerous tissue because it acquires CT scans (e.g. mega-voltage CT or kilo-voltage CT) immediately before, immediately after, or during radiation delivery, with the patient on a treatment apparatus and in the treatment position. This therapy technique uses intensity modulated beams that enter the patient's body at a greater number of angles and positions than conventional therapies, thereby lessening the amount of radiation that healthy tissues are subjected to and concentrating the radiation where it is needed most, at the cancer site(s). Essentially, the radiation field is “sculpted” to match the shape of the cancerous tissue to keep the dose of radiation to healthy tissue near the cancer low.
A radiation treatment plan may be based on a computed tomography (“CT”) image of the patient. As is known in the art, a CT image is produced by a mathematical reconstruction of many projection images obtained at different angles about the patient. In a typical CT scan, the projections are one-dimensional line images indicating the attenuation of the beam by a “slice” of the patient. The actual CT data is held in a matrix wherein each row represents an angle and each column represents a distance. The matrix of data obtained in a CT scan can be displayed as a sinogram as shown in FIG. 1, or reconstructed into a two-dimensional image, as shown in FIG. 2.
In some radiotherapy systems, the oncologist views the cancerous areas on the CT image and determines the beam angles and intensities (identified with respect to the tumor image) which will be used to treat the tumor. In an automated system, such as that disclosed in U.S. Pat. No. 5,661,773, and hereby incorporated by reference, a computer program selects the beam angles and intensities after the physician identifies the tumorous region and upper and lower dose limits for the treatment.
More specifically, the planning images are used to create a 3-D treatment plan of a region of interest. This region of interest is broken down into units called voxels, which are defined as volumetric pixels. Each voxel is then assigned a particular radiation dose depending on what type of tissue or other matter it contains, e.g. cancerous tissue, air, etc.
Normally, the CT image of the patient is acquired substantially before the radiation treatment to allow time for the treatment plan to be prepared. However, the position of organs or other tissue to be treated can change from day-to-day because of a variety of factors. Further, patients move during treatment because of breathing, muscle twitching or the like. Uncertainty in the positioning of the patient with respect to the original CT image can undermine the conformality of the radiation delivery.
Thus, it is highly preferable to verify the treatment plan based on data obtained just prior to the time of treatment. The verification process can be done by techniques that compare the planning image to an image of the patient at the time of treatment.
Unfortunately, the data sets obtained on the day of treatment to be used for preparing the patient model are often incomplete. Patients that are large in size may not fit within the field-of-view (FOV) of the CT machine attached to the therapeutic equipment applying the radiation dose, and may yield an image such as that shown in FIG. 3, which shows only a portion of the image shown in FIG. 1. Not only is there a limited field of view, the data around the edges contains significant artifacts so that the image has an irregular white border and internal values are distorted. Alternatively, only a limited sample size of slices may have been obtained. There may be other limitations that result in the collection of incomplete data sets.
To resolve the problem of limited data sets in which only a portion of an image can be obtained, several scans of the patient may be made at various detector or patient positions, and then combined into a complete set. This has been done by adding together sinogram data, but requires that the imaging apparatus or patient position can be reliably modified accordingly, which is not always possible. Further, the problem of developing artifacts is still present due to the significant degree of mismatch between such data sets, and the additional handling of the patient is more costly, time intensive and can be difficult for frail patients. Moreover, the patients receive a higher dose of radiation with multiple scans than with one single scan.
Reconstruction of incomplete data sets using available techniques results in images that do not show the complete extent of the patient's body, can have artifacts and incorrect voxel values, and thus, limit the extent to which the images can be used for delivery verification, dose reconstruction and patient set-up, deformable patient registration and deformable dose registration. Accordingly, a need exists for a system and method that can solve the problems caused by limited data sets.